1. Field of the Invention
This invention relates generally to fasteners, and more particularly to a fastener which is installed through automated installation and having a shoulder which provides an interference fit with a hole into which the fastener is being installed.
2. Description of the Related Art
Fasteners of the type to which this invention is directed ordinarily include a head at a proximal end, a shoulder distal to the head and a male threaded portion distal to the shoulder. The shoulder is typically sized to provide an interference fit with a hole that is located within a structure into which it is to be installed. The fastener also normally includes an aperture on a distal face of the fastener for receiving a torquing tool. Since the shoulder provides an interference fit, the threaded portion includes a diameter which is less than the diameter of the shoulder to avoid interference between the thread and the hole into which the fastener is to be installed.
At installation, the fastener is advanced distally through the hole until a distal surface on the head of the fastener contacts the structure and the shoulder is engaged with the hole. One method of advancing the fastener through the hole includes inserting the threaded portion of the fastener into the hole from one side of the structure and pushing the head distally toward the structure until the distal surface of the head contacts the structure. An alternative method includes inserting the threaded portion of the fastener into the hole from one side of the structure until the threaded portion is protruding through a surface on the opposite side of the structure. This step is followed by engaging the threads of the fastener with a tool and pulling the fastener distally through the hole until the distal surface of the head of the fastener contacts the structure. Having an interference fit, the fastener is retained within the hole in the structure by compressive forces from the hole being applied to the shoulder of the fastener. With the fastener thus retained, a female threaded device, such as a nut, is torqued onto the threaded portion of the fastener. The initial torquing of the female threaded device onto the fastener is performed lightly to prevent the fastener from spinning within the hole. To increase the level of torque between the female threaded device and the fastener, a driving tool, such as a hex-key or Allen wrench, is commonly inserted into a hexagon shape aperture at the distal face of the fastener and applies torsional force in a first direction, such as a counterclockwise direction, while a driving tool is engaged to the female threaded device to apply torsional force in a second direction, such as a clockwise direction.
Having a hexagon shape aperture at the distal face of the fastener requires that the hex-key be aligned with the aperture prior to insertion into the aperture. Oftentimes, a structure, such as an airframe, includes large quantities of fasteners of this configuration. Eliminating the step of aligning the hex-key on each such fastener in a structure can create time savings, and therefore cost savings, for structure assembly.
Another concern of manufacturers is the weight of a fully assembled structure including the fasteners. A large quantity of fasteners can add significantly to the total weight of the structure. For vehicle structures, such as an airframe, increased weight translates into increased operating costs due to increased fuel consumption and reduced payload. Although the hexagon shape aperture serves to reduce the weight of the fastener, there are structural limitations on the size of a hexagon shape aperture.
An additional concern is the cost of fabricating the hexagon shape aperture within the fastener. Fabricating the hexagon shape aperture commonly requires first adding a circular shape aperture to the fastener, such as by a drilling operation, followed by a broaching operation to transform the circular shape aperture to a hexagon shape aperture. The broaching operation includes forcing an elongated, serrated cutting tool having a hexagonal cross section into the circular shape aperture to enlarge and shape the circular shape into a hexagon shape. The broaching operation often requires an additional machine setup over the initial setup for adding the circular shape aperture. The additional machine setup and additional manufacturing step adds to the manufacturing costs of the fastener.
For the foregoing reasons, those skilled in the art have recognized the need for a fastener which allows manufacturers to save time and costs during the fabrication of the fasteners and the assembly of structures. Further, those skilled in the art have recognized the need to reduce the weight of fasteners. The present invention fulfils these needs and others.